Organic electroluminescent devices (organic EL devices) are composed of a light-emitting layer or plural organic functional layers including a light-emitting layer and a pair of opposing electrodes sandwiching those layers. Organic EL devices are light-emitting devices which utilize light emission from excitons generated by recombination of electrons injected from a cathode and holes injected from an anode in a light-emitting layer, and light emission from excitons of other molecules generated by energy transfer from at least one of the excitons.
Organic EL devices have been developed with a significant improvement in brightness and device efficiency that has been achieved by using laminated structures allowing separation of functions. For example, it is common to use two-layer laminated type devices having a hole-transport layer and a light-emitting electron-transport layer; three-layer laminated type devices having a hole-transport layer, a light-emitting layer and an electron-transport layer; and four-layer laminated type devices having a hole-transport layer, a light-emitting layer, a hole blocking layer and an electron-transport layer (see, for example, Science Vol. 267, No. 3, 1995, page 1332).
However, there are still many problems in practical uses of the organic EL devices. Firstly, a higher efficiency of light emission is desired, and secondly, a higher driving durability is desired. In particular, reduction in quality during continuous driving is the greatest problem to be overcome.
For example, an attempt has been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-123984) in which an external quantum efficiency is raised by providing an interfacial layer of from 0.1 nm to 5 nm as a barrier layer between the light-emitting layer and the hole-transport layer so as to delaying the movement of the holes and so as to adjust a balance of the hole movement and the electron movement. However in the attempt, there is a possibility that reduction of the movement of the carriers as a whole may lead to a reduction in brightness and an increase in the driving voltage, and that prolonged residence time of the carriers in the devices may lead to reduction in the driving durability.
Also, multilayer structures (multiphoton structures) prepared by stacking plural light-emitting units each including a light-emitting layer and a functional layer are known. For example, a constitution has been disclosed (see, for example, JP-A No. 6-310275) in which light-emitting units of plural organic electroluminescent devices (hereinafter also referred to as “organic EL devices”) are separated by an insulating layer, and each light-emitting unit is provided with a pair of opposing electrodes. However in the constitution, because the insulating layer and the electrodes between the light-emitting units inhibit extraction of emitted light, it is substantially true that light emission from each light-emitting unit cannot be utilized sufficiently. Further, the technique does not serve as a measure to improve poor external quantum efficiency inherent in each light-emitting unit.
When the luminescent device is of a polymer dispersed type, the light-emitting layer usually has a monolayer structure and thus the light-emitting sites disperse in the light-emitting layer; therefore it is difficult to keep a balance of injection and transport of holes and electrons, which may cause reduction in recombination efficiency. As a countermeasure to address the problem, it has been proposed (see, for example, JP-A No. 2001-189193) to make the concentrations of both of the light-emitting material and the charge transporting material in the light-emitting layer lower at the anode side but higher at the cathode side thereby causing light emission to concentrate at the region of the cathode. Although the countermeasure is effective against the problem peculiar to the polymer dispersed type luminescent device, the light-emitting region is limited to only a region near the cathode side and the entire light-emitting layer is not utilized effectively for the light emission. Therefore the countermeasure does not provide an enhancement of the total efficiency of light emission.
Further, when the organic EL device is configured to have a laminated structure, carrier injection property is lowered by barriers between individual layers, driving voltage is increased and durability is reduced.
With regard to a countermeasure to reduce the barriers between individual layers, it has been proposed (see, for example, JP-A No. 2002-313583) to provide a gradient of the hole-injection material or the electron-injection material contained in each layer, or a gradient in the concentration of the hole-transport material or the electron-transport material in each layer. In this constitution, the light-emitting material in the light-emitting layer is disposed at a specified region in the light-emitting layer that is a bipolar mixed layer. Even in the constitution, light emission occurs only at the specified region where the light-emitting material is disposed.
Compatibility of both enhanced external quantum efficiency and enhanced driving durability is a significantly important problem in designing a practically useful organic EL device, and improvement thereof has been always demanded.